Drive selection begins with selecting the inverter that meets the application torque requirements of the motor.  The motor torque requirements for the application are converted to motor current using the Torque vs Current model for rotary motors and Force vs Current model for linear motors.  A motor current profile is illustrated below.

Inverter continuous current

Inverter max current

Inverter Current Ratings

All current values in SERVOsoft are "effective" values. Often the word "Peak" and "Max" are used interchangeably throughout the program and the Help. In the context of the program, "Peak Current" and "Max Current" are considered the same, which is the "Maximum Effective Current".

Inverter current ratings can be confusing because not all manufacturers rate their inverters the same.  Inverters have two current ratings, a peak rating and a continuous rating.  The peak rating is the maximum current that can be generated for a brief period of time, usually no more than 5 seconds, typically 0.5 – 1.0 seconds.  The continuous current rating is the maximum current the inverter can produce indefinitely. Some vendors rate their inverter current as RMS continuous, some as RMS peak, and some as peak. The difference can be significant.

RMS indicates the power available from current. For sinusoidal brushless motors, the RMS rating is 70%, (1/√2) of the maximum current rating. So, an inverter that is rated at 10A RMS continuous could be rated at 28.28A Max peak-to-peak (10A x √2 x 2 = 28.28A), even though both drives produce the same amount of current.  The more conservative rating of 10A RMS is used, which is the "effective" value, and the most common among servo system vendors. The following graph best illustrates the differences:

Note that difference can be as large as 2.82 times (√2 x 2 = 2.82).


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